在力的动态平衡问题中,可用图解法求极值,但一般仅限于受力个数只有三个时的情形,当遇到有滑动摩擦力等四个力作用的问题时,可先根据 F_f=μN,将 F_f 与 F_N 合为一个力(F_f 与 F_N 的合力的方向只决定于μ),从而将三个以上力的平衡问题转化为三力平衡问题,最后仍可采用图解法求解。 In the dynamic balance of forces, the extremum can be calculated graphically, but it is generally limited to only when the number of forces is only three. When encountering the problem of four forces such as sliding friction, it can be based on F_f. =μN, F_f and F_N are combined into a force (the direction of the resultant force of F_f and F_N is determined by μ only), so that the balance problem of more than three forces is transformed into the three-force balance problem. Finally, the graphical method can still be used to solve the problem.